WO1997034101A1

WO1997034101A1 - Improvements in or relating to bi-metal lined pipe

Info

Publication number: WO1997034101A1
Application number: PCT/GB1997/000661
Authority: WO
Inventors: Robert George Martin
Original assignee: Coflexip Stena Offshore Limited
Priority date: 1996-03-12
Filing date: 1997-03-11
Publication date: 1997-09-18
Also published as: AU2103897A; GB9605231D0

Abstract

A bi-metal lined pipe and methods of forming such a pipe suitable for spooling onto a reel for marine pipelaying applications, the pipe comprising a carbon steel outer pipe and a liner of corrosion resistant metal, in which the liner is secured along its length to the inner surface of the outer pipe by circumferential fixing means having a circumferential shear strength greater than the circumferential shear stress which would be induced by bending the pipe to a predetermined minimum radius of curvature. The circumferential fixing means securing the liner to the outer pipe includes at least one of the following: adhesive agents selected to provide a circumferential shear strength greater than the circumferential shear stress which will be induced by bending a given combination of outer pipe and liner to a predetermined minimum radius; soldering, brazing or other similar technique to produce a mechanically acceptable bond between the liner and the outer pipe without fusing the bulk material; mechanical interlocking of the liner and outer pipe; solid-phase welding, such as electrical resistance spot welding from the interior of the liner. The circumferential fixing means may be used in combination with known friction-bonding techniques.

Description

"Improvements in or relating to Bi-Metal Lined Pipe"

The present invention relates to improvements in pipe of the type having a metal liner, commonly termed "lined pipe" or "bi-metal lined pipe". Lined pipe is pipe in which a layer of corrosion resistant alloy (CRA) is affixed inside a carbon steel pipe (the "outer pipe"), along the full length thereof, by expanding the liner and/or shrinking the outer pipe or by other applicable processes. The CRA provides corrosion protection when the pipe is used for the transport of fluids, such as in the oil industry. The invention is particularly concerned with providing metal-lined pipes which may be plastically bent in one or more directions. Such bending of pipe is required in certain techniques for laying offshore pipelines, such as in the spooling of pipe onto a reel for laying by reel-lay techniques.

Steel pipe is widely used for the transport of fluids associated with the oil industry. The principal advantages of steel pipe are mechanical strength, high temperature capacity and low cost. The principal disadvantage of steel pipe is poor corrosion resistance. Specialised metals, such as stainless steel and nickel based alloys, have been developed which provide the required corrosion resistance, but these are generally expensive and have reduced mechanical strength. Similarly, plastic materials can provide good corrosion resistance, but have physical properties which limit applications to relatively low temperatures.

The cost of corrosion resistant pipe can be minimised by using the corrosion resistant material as an internal cladding for a standard carbon steel pipe. The nature of the cladding material depends upon the operational requirements of the pipeline: plastics can be employed where the operational temperatures permit, but there is an increasing requirement for corrosion resistance at high temperatures which can only be met by metal linings. Metal liners are generally fixed to the outer pipe either by metallurgical bonding or by friction.

Lined pipe is particularly economical for large diameter pipe with correspondingly thick walls, the cost advantage being reduced for smaller diameter pipes with thinner walls, since the inexpensive steel of the outer pipe comprises a smaller proportion of the total material of the complete pipe. A lined pipe which is optimised for cost will have a thick outer pipe for mechanical strength over a thin liner (or "inner pipe") for corrosion resistance.

Pipelines such as are employed in the oil industry are generally constructed by assembling lengths of pipe ( "pipe-joints" ) , typically forty feet (approximately 12m) long. The preferred method of joining the lengths of pipe is welding, because this provides a joint which has similar properties to the remainder of the pipe, so that the pipeline can be regarded as homogeneous whole.

Offshore pipelaying by the reel-pipelay method involves a continuous length of pipeline being spooled onto a storage reel before being transported to the lay site where it is unspooled from the pipelay vessel. This allows most of the required welding to be carried out onshore at a "spoolbase", under optimal conditions, as compared with conventional "stovepipe" lay methods where the pipeline is assembled joint-by-joint on board a laybarge as the pipe is being laid. Onshore welding is advantageous in terms of cost and quality control, particularly where specialised metals are involved which may significantly increase the welding time as compared with conventional steel.

The vessel CSO Apache (formerly the Apache) is an example of a reel pipelaying ship, and is described in the following US Patents:-

Springett, et al - US Patent No. 4,230,421 Uyeda, et al - US Patent No. 4,269,540 Yenzer, et al - US Patent No. 4,297,054 Springett, et al - US Patent No. 4,340,322 Uyeda, et al - US Patent No. 4,345,855

Large diameter pipes are more difficult to reel than smaller diameters, so that the maximum cost advantage of lined pipes is more difficult to realise when using the reel-lay method.

The process of reeling lined pipe may involve certain difficulties. The liner may respond differently from the outer pipe to bending, particularly where their thicknesses are significantly different (as will normally be the case, for the reasons noted above) . It is known that if the liner is metallurgically bonded to the outer pipe, no separation occurs between the liner and the outer during bending. However, if the liner is secured only by frictional means then separation may occur when the pipe is bent during reeling. Such separation may typically take the form of wrinkling of the liner on the inside face of the bend, where the pipe is in compression. It appears that this wrinkling occurs as a result of the liner tending to slide relative to the outer pipe along its length (axially), but being restrained at the girth welds joining the pipe lengths where the liner is fixedly secured to the outer around its complete circumference by the welding process.

At the time of writing it has not been proven that the bonding of the liner to the outer pipe at the girth weld is the source of this problem, and it may be that an unrestrained liner would also wrinkle during the reeling process. Nevertheless, it is clear that the welding process would be made much more complex if it was necessary for the liner to be free to slide relative to the outer pipe at the location of the weld.

Given that there is known to be a problem with reeling a pipeline comprising conventional lined pipe in which the liner is secured to the outer by friction and in which the pipe joints are connected by conventional welds, such that the liner is bonded to the outer at the welds, three possible approaches to the reeling of lined pipe have been identified as follow:

- "Full liner fixity"; i.e. the liner being fully secured to the outer along its entire length. This is achievable by means of metallurgical bonding, providing good performance but being economically unfeasible for general use. Such pipe is only utilised for specialised applications. (Metallurgically bonded, "clad" lined pipe may be manufactured by a number of methods: longitudinally welded pipe made from a clad plate; centrifugal casting; extrusion of a composite billet; liquid interface diffusion bonding (LIDB); hot isostatic pressing (HIP); explosive bonding; and others) .

- "Partial liner fixity"; i.e. the liner being partially fixed to the outer in a manner which provides good bending/reeling performance.

- "Free liner"; i.e. making the liner free to slide relative to the outer along its entire length. It has not been established that such an approach would be successful and, in any case, this would be very difficult to achieve in practice owing to the complexity of the welding process which would be required.

The present Applicant believes that the wrinkling of the liner adjacent the girth welds may be caused, at least in part, by the ovalisation of the pipe which occurs during the reeling process. During such ovality changes, the liner will attempt to slide circumferentially (rather than axially) within the outer pipe, but will be constrained from doing so by the girth weld which secures it to the outer.

The girth welds at the ends of the pipes are about 10mm wide, and clearly cannot provide full circumferential fixity for the 12 m length of liner between welds. Accordingly, when the pipe is bent there is a transition between the local fixity of the girth weld and the natural sliding action of the remainder of the liner. This will take the form of regions of high shear stress on either side of the weld, together with slightly reduced ovality changes at the weld itself. These effects combined with the axial compression associated with pipe bending will cause sufficient hiatus to initiate local buckling in the liner, as observed adjacent to the girth weld in practice.

Assuming that the transition between circumferential fixity and sliding is responsible, at least in part, for the wrinkling of the liner, then it should be possible to remove, or reduce, the wrinkling effect by removing the transition; i.e. by securing the liner against circumferential sliding movement along its entire length. The strength of the circumferential fixity required between the outer pipe and the liner must be sufficient to resist the circumferential shear stress induced by bending the pipe to a predetermined minimum radius of curvature (i.e. the smallest radius to which it is intended that the pipe will be bent in use). The required circumferential fixity can be calculated by simple beam theory.

The simplest method of supplying such fixity is by friction. This is primarily a function of the contact pressure between the liner and the outer pipe, which in turn is determined primarily by the manufacturing process. Two existing processes for producing "friction bonded" metal lined pipe are:

(a) Cold expansion of the liner into contact with the outer followed by further cold expansion of the liner and outer together. When the expansion pressure is released, the two pipes contract elastically and the residual contact pressure depends on differential contraction between the outer and the liner. This depends on the respective stress/strain properties of the two materials, and for an Inconel liner inside a carbon steel outer the effect is relatively small. The contact pressure can be enhanced by use of a higher grade carbon steel, but this is not necessarily desirable in terms of performance for reeling purposes.

(b) Cold expansion of the liner into contact with an outer pipe which has been heated (typically to about 300°C, the "shrink-fit temperature"). The outer then shrink-fits onto the liner. The residual contact pressure is again a function of the respective stress/strain properties of the materials and also, more importantly, of shrink- fit temperature. This will be limited by the properties of the carbon steel outer.

The shrink-fit technique (b) should, theoretically, give much higher contact pressures than the cold expansion technique (a) . However, the contact pressure is also limited by the strength of the liner. A thinner liner will be able to support less contact pressure before yielding. In practice, the thickness of the liner is not a significant parameter for friction bonding. A thinner liner requires less shear fixity at its interface with the outer, but can also sustain proportionately less contact pressure. The required "friction coefficient" between the two surfaces is therefore broadly unchanged.

A beam-theory analysis allows calculations to be performed to estimate the circumferential shear strength required for a given pipe and liner combination and a given minimum radius of curvature. The nominal pipe diameter, the outer pipe wall thickness, the liner thickness and the properties of the outer pipe and liner materials must all be taken into account in such calculations.

The vessel CSO Apache has reel-hub radius of 8.23 m, allowing pipe of up to 16" diameter to be spooled. Using the above mentioned analytical method, it can be shown that the contact pressure and friction obtainable using friction-bonding methods (particularly the shrink-fit method (b) above) would provide circumferential shear strength sufficient for pipe up to a maximum nominal outside diameter of 8" to be spooled onto the Apache reel . For a 6 " carbon steel pipe having a 15.88mm wall thickness and a 3mm thick Inconel liner, a circumferential shear strength of about 3.2 MPa is required for spooling onto the Apache reel. Shrink-fit fixing provides about 4.1 MPa in a pipe of this type, which is more than adequate. An 8" pipe with 15.88mm wall thickness and 3mm liner requires about 5.1 MPa, which may be barely achievable by shrink-fit techniques. A 16" pipe with 22.23mm wall thickness and 3mm liner requires about 13 MPa, while shrink-fit techniques will provide less than 2 MPa in a pipe of this type, which is clearly inadequate.

The present invention concerns the provision of metal- lined pipe in which the liner is secured to the outer in an economically feasible manner and which provides circumferential shear strength which is higher than that obtained using conventional shrink-fit techniques, thus enabling the successful plastic deformation of metal-lined pipes (such as when spooling pipe on a reel) having an outside diameter greater than has hitherto been possible for a given minimum bend radius. The invention resides in the formation of a bond between the outer pipe and liner which is compatible with the relevant manufacturing and reeling processes, and may include the use of a bonding agent or mechanism in combination, possibly in combination with a conventional friction-bonding process (either cold expansion (a) or, more likely, shrink-fit (b) as discussed above) . The bonding technique must be able to withstand the tensile strains produced in the longitudinal axial direction along the outer/liner interface during pipe bending and must be tolerant of the temperatures encountered during shrink-fitting (if applicable) .

In accordance with a first aspect of the invention, there is provided a pipe comprising a carbon steel outer pipe and a liner of corrosion resistant metal, in which the liner is secured along its length to the inner surface of the outer pipe by circumferential fixing means having a circumferential shear strength greater than the circumferential shear stress which would be induced by bending the pipe to a predetermined minimum radius of curvature.

In accordance with a second aspect of the invention, there is provided method of forming a pipe comprising a steel outer pipe and a liner of corrosion resistant metal, in which a tubular liner is secured along its length to the inner surface of a tubular outer pipe by circumferential fixing means having a circumferential shear strength greater than the circumferential shear stress which would be induced by bending the pipe to a predetermined minimum radius of curvature.

The means for securing the liner to the outer pipe may include: adhesive agents selected to provide a circumferential shear strength greater than the circumferential shear stress which will be induced by bending a given combination of outer pipe and liner to a predetermined minimum radius; soldering, brazing or other similar technique to produce a mechanically acceptable bond between the liner and the outer pipe without fusing the bulk material; mechanical interlocking of the liner and outer pipe; solid-phase welding, such as electrical resistance spot welding from the interior of the liner; combinations of two or more of the above.

Any of the above mentioned securing means may be used in combination with known friction-bonding techniques as discussed above, most preferably the shrink-fit technique.

Mechanical interlocking may be provided by forming fine grooves on the inner surface of the outer pipe and expanding the liner such that its outer surface deforms into the grooves. The grooves would be arranged so as to resist circumferential movement of the liner within the outer pipe; e.g. the grooves may extend longitudinally along the length of the pipe.

The circumferential securing means becomes redundant after the pipe has been laid on the seafloor, so that adhesive bonding agents or other securing means used may be of a type which will deteriorate once the pipe has been laid.

Liner thickness is not particularly significant for frictional fixity of the liner, but is significant for surface bonding where the circumferential shear stress is broadly proportional to the thickness of the liner. Accordingly, it is desirable to minimise the liner thickness. This is advantageous both from the point of view of surface bonding considerations and for reasons of cost. Accordingly, it is preferred that the liner has a thickness less than 3mm (being the current norm for metal liners).

Epoxy is a particularly preferred bonding agent. Epoxy adhesives may provide shear strengths up to 20 MPa, which is more than adequate for spooling a 16" pipe with a 2mm liner onto the reel of the CSO Apache.

Bonding agents may be applied to the outer surface of the liner or the inner surface of the outer pipe, or both, prior to inserting the liner into the outer pipe. Alternatively, the liner may be inserted into the outer pipe prior to pouring the bonding agent into an annular gap between the liner and the outer pipe.

Soldering, brazing or similar techniques may be employed to bond the two surfaces directly. A liquid would be made to flow to fill a space between the two components and then solidify, the liquid having a lower melting point than either the liner or the outer pipe. The solder/brazing medium (i.e. a combination of metal and flux) may be applied to the outer surface of the liner and/or the inner surface of the outer pipe prior to inserting the liner. The assembly would then be heated to allow flow and filling of the joint space by the solder or brazing metal.

The strength of the bond between the outer pipe and the liner may be improved by suitable preparation of the mating surfaces; e.g. cleaning, shot-blasting, the use of primer materials etc. If additional friction bonding is applied by cold expansion (as discussed above, among other possible friction bonding techniques), this may be accomplished by means of swaging tools pulled through the lined pipes, typically using a series of successively larger mandrels. The final mandrel should preferably be dimensioned so as to expand both the liner and the outer pipe together.

Improvements or modifications may be incorporated without departing from the scope of the invention.

Claims

1. A pipe comprising a carbon steel outer pipe and a liner of corrosion resistant metal, in which the liner is secured along its length to the inner surface of the outer pipe by circumferential fixing means having a circumferential shear strength greater than the circumferential shear stress which would be induced by bending the pipe to a predetermined minimum radius of curvature.

2. A pipe as claimed in Claim 1, wherein said circumferential fixing means securing the liner to the outer pipe includes at least one of the following: adhesive agents selected to provide a circumferential shear strength greater than the circumferential shear stress which will be induced by bending a given combination of outer pipe and liner to a predetermined minimum radius; soldering, brazing or other similar technique to produce a mechanically acceptable bond between the liner and the outer pipe without fusing the bulk material; mechanical interlocking of the liner and outer pipe; solid-phase welding, such as electrical resistance spot welding from the interior of the liner.

3. A pipe as claimed in Claim 2, wherein said adhesive agent is an epoxy material.

4. A pipe as claimed in Claim 2, wherein said mechanical interlocking is provided by fine grooves provided on the inner surface of the outer pipe and by expanding the liner such that its outer surface deforms into the grooves.

5. A pipe as claimed in Claim 4, wherein said grooves are arranged so as to resist circumferential movement of the liner within the outer pipe.

6. A pipe as claimed in any one of Claims 2 to 5, wherein said circumferential fixing means is used in combination with friction-bonding.

7. A pipe as claimed in Claim 6, wherein said friction bonding comprises shrink-fit bonding.

8. A method of forming a pipe comprising a steel outer pipe and a liner of corrosion resistant metal, in which a tubular liner is secured along its length to the inner surface of a tubular outer pipe by circumferential fixing means having a circumferential shear strength greater than the circumferential shear stress which would be induced by bending the pipe to a predetermined minimum radius of curvature.

9. A method as claimed in Claim 8, wherein said circumferential fixing means securing the liner to the outer pipe includes at least one of the following: adhesive agents selected to provide a circumferential shear strength greater than the circumferential shear stress which will be induced by bending a given combination of outer pipe and liner to a predetermined minimum radius; soldering, brazing or other similar technique to produce a mechanically acceptable bond between the liner and the outer pipe without fusing the bulk material; mechanical interlocking of the liner and outer pipe; solid-phase welding, such as electrical resistance spot welding from the interior of the liner.

10. A method as claimed in Claim 9, wherein said adhesive agent is an epoxy material.

11. A pipe as claimed in Claim 9, wherein said mechanical interlocking is provided by fine grooves provided on the inner surface of the outer pipe and by expanding the liner such that its outer surface deforms into the grooves.

12. A pipe as claimed in Claim 11, wherein said grooves are formed so as to resist circumferential movement of the liner within the outer pipe.

13. A pipe as claimed in any one of Claims 9 to 12, wherein said circumferential fixing means is used in combination with friction-bonding.

14. A pipe as claimed in Claim 13, wherein said friction bonding comprises shrink-fit bonding.

15. A method as claimed in Claim 9, wherein said adhesive agents are applied to the outer surface of the liner or the inner surface of the outer pipe, or both, prior to inserting the liner into the outer pipe.

16. A method as claimed in Claim 9, wherein the liner is inserted into the outer pipe prior to pouring the adhesive agent into an annular gap between the liner and the outer pipe.

17. A method as claimed in Claim 9, wherein soldering, brazing or similar techniques are employed to bond the two surfaces directly.

18. A method as claimed in Claim 17, wherein a liquid is made to flow to fill a space between the outer pipe and the liner and then solidify, the liquid having a lower melting point than either the liner or the outer pipe.

19. A method as claimed in Claim 17, wherein the solder/brazing medium is applied to the outer surface of the liner and/or the inner surface of the outer pipe prior to inserting the liner prior to heating the assembly to allow flow and filling of the joint space by the solder or brazing metal.